Electric initiator structure



Aug. 1, 1967 R. W. SCHNETTLER ELECTRIC INITIATOB STRUCTURE Filed June 9, 1966 FIG.

ROBERT WS'CHNETTLER I N'VENTOR.

AGENT United States Patent 3,333,538 ELECTRIC INITIATQR STRUCTURE Robert W. Schnettler, Cumberland, Md., assignor to Hercules Incorporated, Wilmington, DeL, a corporation of Delaware Filed June 9, 1966, Ser. No. 556,428 11 Claims. (Cl. 102-28) This invention relates to electric initiator structure characterized by improved high resistance to premature firing by static electric charges and stray currents.

Electric initiators comprise, in general, a shell, a pair of lead wires extending into and terminating within the shell, a resistance, or bridge, wire connecting the terminal ends of the lead wires, and an ignition composition within the shell ignitable in response to heat developed by passage of electric current through the lead and resistance wires so as to, in turn, initiate detonation, or fast burning, of another material, generally, but not necesarily, within the shell. Various types of electric initiators, such as electric blasting caps of both the instantaneous and delay types, squibs, and the like are well known in the art.

Ignition compositions utilized in initiator assemblies of the type above described are highly heat sensitive. Accordingly, a discharge of high static voltage can cause ignition of the said composition and firing of the initiator. The art is aware that such accidental firings can result from a direct discharge from a lead wire in the shell to the shell wall in the locus of the ignition composition. The ignition composition can also be fired by direct high-static discharge through the bridge wire via the lead wires. The initiator can also be fired by the passage of a portion of a static charge through the bridge wire when the discharge is from shunted lead wires to shell at a point other than through the explosive charge. Further, premature firing can be caused by stray, or lower voltage, currents attending many commercial blasting operations.

Various type structures have been proposed wherein one or both of the bared lead wires are connected to the cap shell by a conductive material outside the locus of the ignition composition. However, it is diflicult with this structure to maintain a proper balance of conductivity that will allow a discharge from goth wires to the shell and still have sufficient resistance for protection against stray currents. Additionally, it has been found that in some instances the static resistance of this type of structure diminishes with storage. Another type structure contains a conductive plug in direct contact with the lead wires in the shell which is spaced apart from the inner shell wall. However, this type structure is committed to very close tolerances with reference to its positioning in the shell because of the precise, and small, spacing required. Thus, in the manufacture of this latter type of structure it is difiicult to hold within the tolerances thereby making the assembly highly susceptible to premature firing by static charges and stray currents. Further, in this type structure, multiple sparking adversely affects nonconductivity of the spacing, i.e., the spark-gap, and often the conductive surface is limited in its capacity to carry high energy discharges.

This invention is concerned with electric initiator structure that can be fabricated without introducing susceptibility to premature firing by static charges and stray cur- 3,333,538 Patented Aug. 1, 1967 wall; a pair of electric conductor wires extending into said shell first through said dielectric sheet and then through said plug and terminating within said shell, and a resistance wire within said shel connecting the ends of said conductor wires therein; a plurality of electrically conductive bodies uniformly disposed on the surface of said dielectric sheet facing said plug; all of said conductive bodies being equilateral hexagons of substantially the same size, except for such equilateral hexagons intercepted as described hereinafter, and disposed in a pattern of separate sets of parallel rows; all adjacent hexagons in each row being'equally spaced apart from each other, and from adjacent hexagons in each adjacent row, such that the directly opposing sides of all adjacent hexagons in said pattern are parallel, and equally spaced from each other, whereby all hexagons of said pattern are separated by gaps of substantially the same width; any said conductive body at the end of a row of said bodies and intercepted by said inner shell wall having its shape concomitantly altered, and being in electrical conductive contact with the said wall along the entire resulting line of interception; the shortest distance from each of said electrical conductor wires to said shell wall being on the line of centers of said conductor wires along said dielectric sheet and equal to the sum of the shortest distance between parallel sides of one of said equilateral hexagon conductive bodies and the width of one of said gaps, and the distance between said conductor wires being at least equal to said sum; said pattern of conductive bodies being disposed on said dielectric sheet such that the rows of one of said separate sets of parallel rows are parallel to the above-said line of centers of said electrical conductor wires; and an ignition composition within said shell characterized by voltage breakdown greater than that between each said lead wire and said shell along said dielectric sheet.

The conductive bodies can be formed from any suitable conductive material. Any conductive ink which can be printed on the dielectric surface can be advantageously utilized. Silver ink is now preferred and can easily be applied to the dielectric surface by the silk screen method. Pure metals, particularly those applied by vacuum metallizing or by electroplating can be utilized. Conductor body materials having a maximum resistivity in the order of about 0.5 ohm per square are particularly preferred.

The invention is applied to any electro-explosive device which utilizes an ignition composition in operative communication with a resistance wire assembly for firing.

The invention in a now preferred form is illustrated with reference to the drawings of which FIG. 1 is a crosssectional view, of an initiator assembly of the invention, taken along a line, 1-1 of FIG. 3, intermediate the dielectric plug and the conductive bodies on the dielectric sheet adjacent thereto, and looking towardthe said conductive bodies; FIG. 2 is a cross-sectional view taken along the line 2-2 of FIG. 1, drawn through the centers of the conductor wires, illustrative of contact of the conductive bodies and of positioning of the dielectric sheet with reference to the bared lead wires and inner shell wall; FIG. 3 is a cross-sectional view of a now preferred initiator embodiment of the invention; and FIG. 4 is the same as FIG. 1 except that the dielectric sheet and conductive bodies associated therewith are disposed on only a central portion of the dielectric plug surface.

Referring to FIG. 1, electrically conductive bodies 10 are equilateral hexagons of substantially the same size except for those intercepted by the inner shell wall as described more fully hereinafter. Bodies 10 are preferably formed from a conductive ink applied to dielectric sheet 11 (see also FIGS. 2 and 3) in accordance with the silk screen method. Bodies 10 are of any suitable conductivity to conduct high energy discharges.

Conductive bodies are uniformly disposed on the surface of dielectric sheet 11. All of hexagonal bodies 10 on sheet 11 are of the same shape except as described hereinafter and are disposed in a pattern of separate sets of parallel rows, viz. rows A, B, and C of a first set, rows A, B, and C, of a second set, and rows A", B", and C of a third set. All hexagonal conductive bodies 10 in each row are equally spaced apart from each other and from adjacent hexagonal conductive bodies in each adjacent row such that the directly opposing sides of all adjacent hexagons in said pattern are parallel to, and equally spaced apart from, each other by a gap of substantially the same width. By way of illustration the hexagon 10b of row B is adjacent to hexagons b and b" of row B and equally spaced from those hexagons; hexagon 10b is adjacent to and equally spaced from hexagons 10a and 10a of row A and is adjacent to and equally spaced from hexagons 10c and 100 of row C, and the directly opposing sides of all these adjacent hexagons, e.g., sides 1 and 1 of hexagons 10b" and 10a are parallel to each other and thereby equally spaced apart to provide one of gaps 12, all which are of substantially the same width.

All conductive bodies 10 are of substantially the same shape except that due to extension of each of the rows, e.g. at least one of rows A, B, or C, toward the shell wall, the conductive body at the end of the row may be intercepted by the shell inner wall with concomitant alteration in its shape, e.g. partial hexagonal conductive body 1011", and being in direct electrical contact with the shell inner wall along the entire line of interception.

Conductor lead wires 18 extend into shell 9, see FIG. 3, and then through dielectric sheet 11 and conductive bodies 10 disposed thereon and then through plug 8 as described more fully hereinafter.

Wires 18 as positioned intermediate dielectric sheet 11 and plug 8, are each spaced apart from the shell inner wall so that the shortest distance from each positioned wire to the shell wall is along the line of centers of wires 18 and is equal to the sum of the Width of a gap 12 and the shortest distance between parallel sides of an equilateral hexagonal conductive body 10. The distance between wires 18 is equal at least to the shortest distance from each wire 18 to the shell 9 as above described.

The pattern, above described, consists inherently of three sets of intersecting rows of conductive bodies 10, as described above. The pattern of bodies are disposed on dielectric sheet 11 such that the parallel rows of one of the three sets are parallel also the line of centers of conductive wires 18.

With reference to FIG. 2 bared conductor wires 18 extend into electrically conductive shell 9, first through dielectric sheet 11 and then through ignition plug 8. Dielectric sheet 11 is superposed on, and completely covers, dielectric plug 8. Dielectric sheet 11 extends to the inner wall of shell 9 along its entire periphery. Hexagonal conductive bodies 10 on the traversely extending surface of sheet 11 facing dielectric plug 8 are in superposed contact with dielectric plug 8. Generally a plurality of conductive bodies 10 is intercepted at the periphery of the sheet surface so as to be in electrical contact with the shell wall along the entire resulting line of interception. In the particular design illustrated, see FIG. 1, there are twelve such intercepted conductive bodies shown.

FIG. 2 also illustrates one aspect of a now preferred method for manufacture of the assemblies of the invention with reference to the insertion of the dielectric sheet 11. As shown, sheet 11 extends along its entire periphery not only to the shell inner Wall but also for a short distance upwardly along the said inner wall. This is preferred practice in order to further assure firm electrical contact of the peripherally disposed conductive bodies 10 with the inner wall of shell 9. This feature is accomplished by cutting a sheet 11 of size slightly greater than necessary for closing the shell 9 and then inserting the sheet in position. The resulting stress of the sheet against the shell wall assures positive electrical contact of the conductive bodies with the shell wall as described above. As is generally the case, each of the bared lead wires 18 is in contact completely or at least in part, with a conductive body 10 at the points where the lead Wires extend from the dielectric sheet 11 into dielectric plug 8.

With reference to FIG. 3, metallic shell of blasting cap assembly 21 is substantially cylindrical and is closed at bottom end 22 and contains dielectric ignition plug 8 spaced from bottom end 22 as a closure for shell 9. Ignition plug 8 is coaxial with shell 9. Superposed on the surface of plug 8 transverse to the interior of shell 9 and farthest from shell end 22 is dielectric sheet 11. Sheet 11 completely covers the surface of ignition plug 8 adjacent thereto and extends to the inner wall of shell 9 along its entire periphery as described with reference to FIG. 2. Also as illustrated with reference to FIG. 2 are hexagonal conductive bodies 10 on the surface of sheet 11 facing plug 8 and in contact with plug 8.

Electrical conduitor wires 18 connected with leads 29 by connectors 30 extend into shell 9 through the upper end 26 thereof, first through dielectric sheet 11 and then through the ignition plug 8, terminating intermediate plug 8 and the shell end 22. Conductor wires 18 can be insulated as desired except that the portions thereof extending from sheet 11 to dielectric plug 8 are bare so as to afford operative electrical contact with the conductive bodies as described with reference to FIG. 2.

Although ignition plug 8 can serve as a suitable waterproofing and sealing plug to close shell 9, it is generally important that an additional dielectric seal be placed above dielectric sheet 11 to furnish the necessary protection, i.e. waterproofing and the like, for the said sheet 11. Thus, plug closure 28, as, for example, a suitable injected plastic material, can complete the upper end of the assembly 21. This closure member also affords further support and electrical insulation for lead wires 18 and connectors 30 as they extend into shell 9.

The terminal ends of lead wires 18 in shell 9 are connected by bridge wire 27. The ignition composition 31 is a loose ignition powder such as lead-selenium and is disposed intermediate shell end 22 and dielectric plug 8 in direct contact with bridge wire 27. The ignition composition 31 is ignitable in response to heat developed by passage of electric current through bridge wire 27 via leads 18. Delay fuse element 32 is a longitudinally perforated metal carrier 33, e.g. lead, containing a suitable delay fuse composition 34 in the perforation, e.g. barium peroxide-tellurium, and is disposed intermediate ignition composition 31 and shell end 22 in direct contact with ignition composition 31. Fuse composition 34 is ignitable in response to heat developed by burning of composition 31. The selection of the length, and the composition of, the delay fuse 34 determines the time, or delay, intermediate to burning of the ignition composition 31 and detonation of primer composition 36 described below. When desired, delay fuse assembly 32 can be dispensed with for instantaneous firing.

Detonatable charge 36 intermediate closed shell end 22 and delay fuse assembly 32 is in direct contact with fuse 32. Charge 36 serves as a primer, e.g. diazodinitrophenol, to detonate in responseto heat from burning of fuse 34. Base charge 37 which occupies the remaining portion of shell 9, i.e. intermediate shell end 22 and primer 36 is in direct contact with primer 36 and is detonatable in response to detonation of primer 36.

With reference to FIG. 4 is illustrated the embodiment of FIG. 1 except that the dielectric sheet-conductor body structure extends across, and is superposed on, only a central portion of the dielectric plug, i.e. instead of completely covering the plug. Thus, equilateral hexagon conductive bodies '10 and 10" are uniformly disposed on dielectric sheet 11' which extends across central portion XX of dielectric plug 8 to the inner walls of conductive shell 9'. Conductor bodies 10' and 10" are disposed in separate sets of parallel rows in the same manner as described with reference to bodies of FIG. 1, and bear the same relationship to adjacent conductive bodies as described with reference to conductive bodies 10 of FIG. 1. Hexagonal conductors 10' at the end of the rows of conductors extending to the inner shell wall 9 and intercepted by the shell wall, concomitantly become partial hexagons in electrical contact with the inner shell wall 9' as described with reference to contact of electrical conductive bodies 10 with shell wall 9 of FIG. 1. The hexagons adjacent the sides 11 of sheet 11 are similarly terminated along the edges 11" of sheet 11. All other 7 relationships among conductive bodies 10 and 10", conductor wires 18' and inner shell wall 9' are the same as those for corresponding elements of FIG. 1 as described above.

It is to be understood that when referring herein to the shell wall of the initiator assembly of the invention, it is meant that the entire wall is electrically conductive. Thus, in accordance with preferred practice, the said shell wall is formed from a metal, often bronze.

The width of the spark-gaps 12 determines the resistance of the path for the static charge to follow in lieu of its passage through the ignition mixture, and thus the spark-gap must be of such dimension as to have a breakdown potential lower than that of the ignition composition. At the same time, if the voltage breakdown is unduly low, the resistance to flow of stray currents is insufficient for suitable protection against premature firing from that source. It is accordingly required that the size of the spark-gap be controlled precisely in order that the voltage breakdown within the requisite range be maintained, for example, from at least 500, and preferably a minimum of 800, to not more than 1500 volts, which is particularly advantageous when utilizing loose ignition mixture compositions as, for example, lead-selenium, leadtellurium, and the like.

To the extent that precision in the formation of sparkgaps in the manufacture of any assembly containing a spark-gap-conductive body unit is faulty, the assembly is susceptible to premature firing by static charges or stray currents and is not suitable from the operational and safety standpoints. In the manufacture of spark-gap type assemblies of the prior art, the spark-gap must be constructed in association with very close tolerances with the result that in many instances production is not commercially feasible due to the susceptibility of the assemblies to faulty spacings and, thus, to premature firing.

The initiator assemblies of the invention are, by virtue of the pattern of conductive bodies and associated spaced apart relationships, never susceptible to erratic spacings with concomitant premature firing. There can be no error associated with fabrication of the assembly that leads to faulty spark-gap structure. There is no spark-gap spacing dependent upon placement of individual components within the shell. It is only necessary that the conductive bodies be affixed to a sheet of dielectric material in a separate and independent step such as by silk screen printing, followed by punching a portion from the thus-processed sheet directly over the conductor wires, previously emplaced in the dielectric plug and then onto the dielectric plug element to provide the dielectric sheet-conductive body element of the device of the invention. This spacing which involves merely a placement of the desired pattern of conductive bodies on the main sheet, punching directly onto the dielectric plug with the line of centers of the conductor wires parallel to a set of rows of conductive bodies requires no alignments of parts other than to maintain the above parallel relationship between the line of centers of the conductor wires and a row of conductor bodies, and results in a placement of the conductive bodies to provide spacings within accuracy range of about 0.002 inch and, hence, eliminates problems associated with the prior art methods that result in marked susceptibility of the units to premature firing.

It is not important whether a bared lead wire touches a conductive body partially or completely inasmuch as the total gap spacing between each wire and the inner shell wall will always be equal to that of -a single gap 12.

The accurately controlled spark-gaps of the initiator assembly of the invention eliminate variances due to fabrication errors that result in premature firing by stray currents when the gap is unduly narrow and by static changes when the gap is unduly large.

There is a plurality of spark-gaps in parallel between the bared lead wires and the shell wall as well as a uniquely large amount of conductive surface, i.e. on the plurality of conductive bodies. This feature renders high static charge carrying capacity and high spark-gap stability after multiple sparking. Accordingly, the initiator assembles of the invention are extremely insensitive to static electric charge, e.g. up to 40,000 volts and higher, and often as high as 100,000 volts at 3,000 mmf. as compared with about 25,000 volts at 750 mrnf. of the prior art assemblies.

The width of the spark-gaps is dependent to a large extent upon the particular ignition composition and the range of breakdown potentials required. Generally, the spark-gap is in the order of from 0.005 to 0.011 inch, which affords protection against stray currents, and which is particularly applicable when the voltage breakdown of the ignition mixture is about 1800 volts or higher.

In now preferred practice the hexagonal conductive bodies are spaced apart to provide spark-gaps of from 0.006 to 0.010 inch in width and the minimum distance between parallel sides of the hexagons is in the range of from 0.060 to 0.064 inch. In all events, the sum of the two dimensions is equal to the shortest distance from the shell wall to the closest conductor wire and at least equal to the distance between the lead wires as above described.

The assembly of the invention is, due to the unique distribution of the spark-gaps and the conductive bodies, conveniently fabricated in accordance with steps involving a printed circuit. Thus, the regular pattern of hexagons and spacings is printed on a suitable plastic material using the silk screen method which can reproduce theshapes to within a:0.0002 inch of that desired, thereby providing for high precision in emplacement of the hexagonal conductive bodies.

The silk screen method lays down a heavy layer of conductive ink which provides a highly conductive path across the hexagon. The pattern is advantageously printed on a continuous sheet of resin-coated plastic, say a strip 7 inches by feet with the spacing and conductive body dimensions selected as above described. The pattern is printed with one of the aforementioned sets of parallel rows being disposed perpendicular to the length of the strip. The resulting sheet is then slit into narrow strips for punching out and placing the discs on the dielectric plug face, the conductor wires having been previously emplaced in the dielectric plug. The strip is fed into a punch assembly and punched out in form of individual discs and each disc is forced over the conductor wires of a dielectric plug whose conductor wires are on a line of centers perpendicular to the length of the strip. The wires pierce the dielectric sheet as it is forced against the dielectric plug face, which results in a firm electrical con tact of the electrical wires with the conductive pattern disposed between the plug face and the dielectric sheet. The punched-out printed circuit element is larger in crosssection than the shell into which it is inserted so that it is, when pressed into the shell, in forced, tight electrical contact with the shell wall. This causes the printed circuit assembly to extend slightly upwardly along the shell at the periphery to assure the uniform electrical contact. In a cylindrical shell assembly the diameter of the punched-out element is usually from about 0.002 to 0.006 inch greater than that of the inside shell diameter.

The invention is further illustrated with reference to the following examples:

Several electric initiating assemblies containing the dielectric sheet-conductive body unit in accordance with the invention, and particularly as illustrated with reference to FIG. 1, were made up and tested for static resistance and voltage breakdown. In each assembly the solid dielectric plug (Bakelite) was cylindrical and 0.264 inch in diameter. A dielectric disc formed from epoxycoated Mylar, 0.007 inch thick, having a diameter of 0.268 inch and containing uniformly distributed and spaced apart equilateral hexagonal shaped bodies of silver ink, of 0.5 ohm per square resistance, on one surface was afiixed to the end of the dielectric plug farthest from the bridge wire with the silver hexagon-containing surface facing, and in direct contact with dielectric plug surface.

Each hexagon was spaced apart from each adjacent one to provide spark-gaps of 0.008 inch each and had a minimum dimension across its surface of 0.062 inch, i.e. between parallel sides. The hexagons were of a thickness of 0.001 inch.

A pair of steel lead wires, 0.025 inch diameter, extended through the centralmost portion of the plug and disc assembly on 0.100 inch centers, protruding from the plug 0.125 inch, leaving a distance of 0.075 inch between the steel wires and a distance of 0.070 inch from a peripheral portion of the plug surface to the lead wire closest thereto. A nickel-chromium alloy resistance wire of about 0.0016 inch in diameter connected the terminal ends of the conductor leads.

A cylindrical bronze shell, 0.250 inch in diameter, closed at one end, was charged with its components as shown hereinafter in the conventional order, i.e., from the bottom of the shell toward the open end, of base charge, primer charge, delay (if present), secondary ignition (if present), and ignition. The plug assembly was then inserted, bridge wire first, into the shell through the open end with necessary force to expand the shell to conform with the dimensions of the plug when emplaced. The plug and disc assembly in each instance was inserted completely within the shell into direct contact of the plug with the ignition mixture. The disc, due to its diameter, was thereby pressed in direct contact along its entire periphery with the metal shell wall, and conductive bodies on the periphery of the disc were intercepted by the shell wall and were disposed in electrical contact along the entire line of their interception.

Tinned copper lead Wires were then attached to the /8 inch steel wires extending from the plug surface with brass connectors. Polypropylene plastic was then injection molded into the shell above the dielectric disc to afford insulation to the bare wires and connectors and to furnish protection to the dielectric disc.

The completed cap assemblies and associated data are further summarized in the following tabulation:

Each of the above-described assemblies was tested for static resistance and voltage breakdown. The results are summarized as follows:

STATIC RESISTANCE TESTS No. Tested Capacitance, Shots Voltage, nunf. V.

Cap Assembly:

Cap A Cap A. Cap B Cap B. Cap C. Cap C Control: Gap A 1 1 Without spark-gap and hexagonal metal structures VOLTAGE BREAKDOWN TESTS, SHUNT T0 SHELL Voltage Breakdown No. Tested Average, Minimum, Maximum,

V. V. V.

Cap Assembly:

Cap A 1, 140

The above examples demonstrate the high resistance to static charges provided in accordance with the invention, 100 percent protection against premature firing by static charges even at the high voltage levels as high as 100,000 at 3,000 m.rn.f., and voltage breakdowns in the range of from 500-1550 volts.

Ten caps the same as cap assembly B, above, except that they contained a slightly shorter delay column, were tested for resistance to accidental firing by stray currents. The lead wires for each cap were shunted and a 440 volt AC source was connected to the cap shell and to the shunted lead wire ends, and the total voltage was impressed between the shunted lead wires and shell for a period of five seconds. In each test the cap did not shoot.

As will be evident to those skilled in the art, various modifications can be made or followed, in the light of the foregoing disclosure and the discussion, without departing from the spirit or scope of the disclosure or from the scope of the claims.

What I claim and desire to protect by Letters Patent is:

1. An electric initiator assembly which comprises a shell having electrically conductive walls; a dielectric plug within, and extending across, said shell to close same; a dielectric sheet disposed on, and covering, at least a central portion of a surface of said plug transverse to the interior of said shell and extending entirely across said plug to the shell inner wall; a pair of electric conductor wires extending into said shell first through said dielectric sheet and then through said plug and bared at least Cap Assembly A Bronze Shell, inches i.d. x length Base charge, PETN, Grams... Primer Charge, Diazo, Grams.

None

Ignition charge:

Secondary Pb-Sn (85-15) SeIBaO /Al/Diat. Earth Grams Shell length (space), inches Primary:

Pb/Se/Diazo /Graphite/Diat. Earth Pb/Se/Diazo /Diat. Earth 2 (wt) Pb/Se/Diat. Earth 2 (wt.)

Grams Shell length (space), inches l Diazodinitrophenol. Diatomaceous Earth.

along their lengths intermediate said sheet and said plug, and terminating within said shell; a resistance wire within said shell connecting the terminal ends of said conductor wires therein; a plurality of electrically conductive bodies uniformly disposed on the surface of said dielectric sheet facing said plug; all of said conductive bodies being equilateral hexagons of substantially the same size, except for such equilateral hexagons intercepted as described hereinafter, and disposed in a pattern of separate sets of parallel rows; all adjacent hexagons in each row being equally spaced apart from each other, and from adjacent hexagons in each adjacent row, such that the directly opposing sides of all adjacent hexagons in said pattern are parallel, and equally spaced from each other, whereby all hexagons of said pattern are separated by gaps of substantially the same width; any said conductive body at the end of a row of said bodies and intercepted by said inner shell wall having its shape concomitantly altered, and being in electrical conductive contact with the said wall along the entire resulting line of interception; the shortest distance from each of said electrical conductor wires to said shell wall bein on the line of centers of said conductor wires along said dielectric sheet and equal to the sum of the shortest distance between parallel sides of one of said equilateral hexagon conductive bodies and the width of one of said gaps, and the distance between said conductor wires being at least equal to said sum; said pattern of conductive bodies being disposed on said dielectric sheet such that the rows of one of said separate sets of parallel rows are parallel to the above-said line of centers of said electrical conductor wires; and an ignition composition within said shell characterized by voltage breakdown greater than that between each said lead wire and said shell along said dielectric sheet.

2. A device of claim 1 wherein said dielectric sheet covers the entire surface of said dielectric plug.

3. A device of claim 1 wherein said dielectric sheet covers a central portion of the surface of said dielectric plug.

4. A device of claim 1 wherein said conductive body is a conductive silver composition.

5. A device of claim 2 wherein said shell is cylindrical and metallic, the width of each of said gaps is within the range of from 0.005 to 0.011 inch, and the voltage breakdown of the ignition composition is at least 1800 volts.

6. An assembly of claim 2 wherein said shell is cylindrical, the diameter of said dielectric sheet is greater than the diameter of the interior of said shell, and a peripheral portion of said dielectric sheet extends upwardly along the inner shell wall under stress.

7. An assembly of claim 2 wherein the entire portion of said dielectric sheet surrounding each of the said conductor wires extends upwardly along the said wires under stress to support electrical contact of each said wire with at least one of said conductive bodies.

8. An assembly of claim 2 wherein a conductive body at each end of each of the said rows is intercepted by the shell wall and is in direct contact therewith along the entire line of interception.

9. In an assembly of claim 2, a delay fuse, a primer charge, and a base charge disposed in that order away from said ignition composition, to form a resulting electric blasting cap assembly of the delay type.

10. In the manufacture of an electric initiator assembly comprising a shell, a dielectric plug, within, and transversely closing, said shell, electrical conductor wires extending into said shell and through said plug and terminating within said shell; a resistance wire connecting the terminal ends of said conductor wires within said shell, an ignition composition within said shell in operative contact with said resistance Wire to ignite in response to heat developed by passage of electric current passed through said resistance wire, and an explosive material within said shell actuatable in response to ignition of said ignition mixture, the improvement comprising printing a pattern, as described hereinafter, of conductive bodies on the surface of a sheet of dielectric material and then cutting a strip from the resulting printed material; disposing a pair of conductor wires within said dielectric plug so as to extend therethrough and protrude from each end of said plug, and both said wires protruding bare from at least one and the same end of said plug; punching a portion of said printed strip, pattern-bearing face first, over one set of protruded portions of said bare conductor wires and then on to said plug, and connecting the other protruded conductor wire ends with a re sistance wire; partially filling said shell with an ignition composition and an explosive, actuatable in response to ignition of said ignition composition, in the inverse order named, and said ignition composition having a voltage breakdown of at least 1800 volts; inserting the resulting plug assembly completely within said shell so as to place said resistance wire in direct contact with said ignition composition; as said pattern, a plurality of electrically conductive bodies uniformly disposed on the said surface of said dielectric material, all of said conductive bodies being equilateral hexagons of substantially the same size and disposed in a pattern of separate sets of parallel rows, all adjacent hexagons in each roW being equally spaced apart from each other and from adjacent hexagons in each adjacent row such that the directly opposing sides of all adjacent hexagons in said pattern are parallel and equally spaced from each other, whereby all hexagons of said pattern are separated by gaps of substantially the same width; prior to punching, as above described, aligning the said strip so that its length is perpendicular to the line of centers of the first said set of protruding ends of conductor wires; printing said pattern so that when said strip is punched, the parallel rows of one of such sets are parallel to the above-said line of centers, and so that the shortest distance from each lead wire to the inner wall of said shell is equal to the sum of the Width of one of said gaps and the shortest distance between parallel sides of one of said hexagons; and positioning said conductor wires within said dielectric plug so that they are separated, as measured along the surface of said dielectric plug, by a distance at least equal to the above-said sum.

11. A method of claim 10 wherein the said dielectric material is punched so as to extend across a central portion of said dielectric plug.

References Cited UNITED STATES PATENTS 2,887,054 5/1959 Bryan 10228 2,974,590 3/1961 Ramer 102-28 3,194,160 7/ 1965 Spillane et .al. 10228 BENJAMIN A. BORCHELT, Primary Examiner. V. R. PENDERGRASS, Assistant Examiner,

UNITED STATES PATENT OFFICE CERTIFICATE. OF CORRECTION Patent No. 3 ,333 ,538 August 1 1967 Robert W. Schnettler It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1, line 43, for "goth" read both column 2, line 4, for "shel" read shell column 4, line 20, for "conduitor" read conductor line 62, for "responseto" read response to column 6, line 8, for "changes" read charges line 16, for "assembles" read assemblies line 43 for 0.0002 inch" read 0 .002 inch columns 7 and 8, in the table at the bottom 5f the page, first column, line 4 thereof, for "Delay charge x read Delay charge same table, first column, lines 10 to 12 should be indented under the word "Secondary" in line 9; same table, same column, line 13 thereof, for "Primary:" read Primary Signed and sealed this 16th day of July 1968.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

1. AN ELECTRIC INITIATOR ASSEMBLY WHICH COMPRISES A SHELL HAVING ELECTRICALLY CONDUCTIVE WALLS; A DIELECTRIC PLUG WITHIN, AND EXTENDING ACROSS, SAID SHELL TO CLOSE SAME; A DIELECTRIC SHEET DISPOSED ON, AND COVERING, AT LEAST A CENTRAL PORTION OF A SURFACE OF SAID PLUG TRANSVERSE TO THE INTERIOR OF SAID SHELL AND EXTENDING ENTIRELY ACROSS SAID PLUG TO THE SHELL INNER WALL; A PAIR OF ELECTRIC CONDUCTOR WIRES EXTENDING INTO SAID SHELL FIRST THROUGH SAID DIELECTRIC SHEET AND THEN THROUGH SAID PLUG AND BARED AT LEAST ALONG THEIR LENGTHS INTERMEDIATE SAID SHEET AND SAID PLUG, AND TERMINATING WITHIN SAID SHELL; A RESISTANCE WIRE WITHIN SAID SHELL CONNECTING THE TERMINAL ENDS OF SAID CONDUCTOR WIRES THEREIN; A PLURALITY OF ELECTRICALLY CONDUCTIVE BODIES UNIFORMLY DISPOSED ON THE SURFACE OF SAID DIELECTRIC SHEET FACING SAID PLUG; ALL OF SAID CONDUCTIVE BODIES BEING EQUILATERAL HEXAGONS OF SUBSTANTIALLY THE SAME SIZE, EXCEPT FOR SUCH EQUILATERAL HEXAGONS INTERCEPTED AS DESCRIBED HEREINAFTER, AND DISPOSED IN A PATTERN OF SEPARATE SETS OF PARALLEL ROWS; ALL ADJACENT HEXAGONS IN EACH ROW BEING EQUALLY SPACED APART FROM EACH OTHER, AND FROM ADJACENT HEXAGONS IN EACH ADJACENT ROW, SUCH THAT THE DIRECTLY OPPOSING SIDES OF ALL ADJACENT HEXAGONS IN SAID PATTERN ARE PARALLEL, AND EQUALLY SPACED FROM EACH OTHER, WHEREBY ALL HEXAGONS OF SAID PATTERN ARE SEPARATED BY GAPS OF SUBSTANTIALLY THE SAME WIDTH; ANY SAID CONDUCTIVE BODY AT THE END OF A ROW OF SAID BODIES AND INTERCEPTED BY SAID INNER SHELL WALL HAVING ITS SHAPE CONCOMITANTLY ALTERED, AND BEING IN ELECTRICAL CONDUCTIVE CONTACT WITH THE SAID WALL ALONG THE ENTIRE RESULTING LINE OF INTERCEPTION; THE SHORTEST DISTANCE FROM EACH OF SAID ELECTRICAL CONDUCTOR WIRES TO SAID SHELL WALL BEING ON THE LINE OF CENTERS OF SAID CONDUCTOR WIRES ALONG SAID DIELECTRIC SHEET AND EQUAL TO THE SUM OF THE SHORTEST DISTANCE BETWEEN PARALLEL SIDES OF ONE OF SAID EQUILATERAL HEXAGON CONDUCTIVE BODIES AND THE WIDTH OF ONE OF SAID GAPS, AND THE DISTANCE BETWEEN SAID CONDUCTOR WIRES BEING AT LEAST EQUAL TO SAID SUM; SAID PATTERN OF CONDUCTIVE BODIES BEING DISPOSED ON SAID DIELECTRIC SHEET SUCH THAT THE ROWS OF ONE OF SAID SEPARATE SETS OF PARALLEL ROWS ARE PARALLEL TO THE ABOVE-SAID LINE OF CENTERS OF SAID ELECTRICAL CONDUCTOR WIRES; AND AN IGNITION COMPOSITION WITHIN SAID SHELL CHARACTERIZED BY VOLTAGE BREAKDOWN GREATER THAN THAT BETWEEN EACH SAID LEAD WIRE AND SAID SHELL ALONG SAID DIELECRIC SHEET. 